enhance modularity and add python bindings

examples/alpm/pactree.vala

examples/python/empty_transaction.py

examples/python/list_installed_pkgs.py

examples/python/list_installed_pkgs_async.py

resources/pamac.manager.gresource.xml

resources/pamac.transaction.gresource.xml

src/clean_cache.vala

src/cli.vala

src/common.vala

src/common_daemon.vala

src/database.vala

src/installer.vala

src/manager.vala

src/manager_window.vala

src/meson.build

src/pamac_config.vala

src/preferences_dialog.vala

src/system_daemon.vala

src/transaction-cli.vala

src/transaction-gtk.vala